In the context of a future-generation high-resolution observation instruments, there is a need to increase the resolution and the associated performance while reducing their bulk. The image-quality performance concerned is the modulation transfer function commonly designated by the acronym “MTF” and location. For the rest of this description, the term “instruments” means all the devices forming a space telescope, that is to say the telescope itself, at least one focal plane and optomechanical devices or specific measuring devices.
The MTF is representative of the quality of the image produced by the instruments and improving it involves corrections. The location is representative of the capability of the instruments to determine the coordinates of a target point in a frame of reference of the satellite. Improving the location simply involves a good knowledge of the latter.
Improving the resolution involves producing a larger telescope pupil involving mirrors with larger diameters. Really, the mirrors used are increasingly sensitive having an increasingly greater relative thinness and consequently are less stable. In parallel, for the purposes of economy, the launches have to be ever smaller, involving a need for instruments that are increasingly compact. This phenomenon further increases the sensitivity of the instruments.
The increased sensitivity of the instruments causes their performance to drift, notably over the long term, and typically means that the latter must be able to be readjusted in flight. Moreover, the increased sensitivity of the instruments makes the latter greatly subject to the variations of thermal flux on the orbital scale, bringing with it major performance instabilities, notably in terms of MTF and location. The thermoelastic variations originate from the flux variations that the telescope cavity undergoes: of terrestrial, spatial or solar origin, according to what the cavity undergoes. The impact of these effects on image-quality performance may be intense, particularly for compact telescopes in which the inter-lens distance is reduced, or else for deployable telescopes because of their greater structural instability.
According to known techniques, the instruments can be designed so as to have a compatible sensitivity—hence a bulk—of the allocated instabilities. In the same manner, the technologies for producing the instruments can be chosen for their compatibility with these instabilities.
For the purpose of making the instruments more compact in order to reduce the launch costs, while increasing their performance, known techniques also make it possible to correct the defects when the satellite is in orbit. These techniques are used in instruments known as “active”. An active instrument typically comprises a device for measuring the defects, correction devices, such as devices moving mirrors, or the deformation of the optical surface, etc. The correction cycle depends on the frequency of change of the defects.
The defects associated with the changes of environment between the ground and flight, such as the effects associated with gravity or the effects associated with the launch loads are to be corrected at the beginning of orbital life. The defects associated with the long term change in flight, notably due to the phenomena of radiation and to aging are to be corrected over long, typically annual, cycles.
The short-term effects on the orbital scale require a complex, rapid correction loop, carrying out a virtually real-time closed-loop control. This rapid correction loop must in a few tens of seconds measure the defects, calculate the correction to be applied and correct the defects, while performing the nominal mission of the telescope which consists in taking images.
The short-term effects are essentially of thermoelastic origin.